Tri-Gate Field-Effect Transistors Formed by Aspect Ratio Trapping

ABSTRACT

Semiconductor structures include a trench formed proximate a substrate including a first semiconductor material. A crystalline material including a second semiconductor material lattice mismatched to the first semiconductor material is formed in the trench. Process embodiments include removing a portion of the dielectric layer to expose a side portion of the crystalline material and defining a gate thereover. Defects are reduced by using an aspect ratio trapping approach

RELATED APPLICATION

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 12/856,402, filed on Aug. 13, 2010, entitled“Tri-Gate Field-Effect Transistors Formed by Aspect Ratio Trapping,”which is a divisional of and claims the benefit of U.S. patentapplication Ser. No. 11/861,931, filed on Sep. 26, 2007, entitled“Tri-Gate Field-Effect Transistors Formed by Aspect Ratio Trapping,”which claims the benefit of and priority to U.S. Provisional ApplicationSer. No. 60/847,424 filed Sep. 27, 2006; the entire disclosures of theseapplications are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

This invention relates generally to semiconductor processing andparticularly to the formation of tri-gate field-effect transistors.

BACKGROUND

Due to the increasing difficulty in shrinking complementarymetal-oxide-semiconductor (CMOS) transistor gate lengths whilesimultaneously controlling leakage current, the traditional single-gatemetal-oxide-semiconductor field-effect transistor (MOSFET) structure(where the inversion layer is formed only in the plane of the wafersurface) may be supplanted by dual- or triple-gate MOSFET structures.These structures, by increasing the gate's control of the channelpotential, allow greater ability to turn off MOSFETs with ultra-shortchannel lengths. Of the various multi-gate MOSFETs structures exploredin recent years, the most promising in terms of manufacturability andperformance are typically variations of the so-called “FinFET”structure; these variations are known by such terms as “Tri-gate,”“Omega-FET,” or “MuGFET.” Generally, for each of these device types, astrip or “fin” of silicon (Si) is formed; subsequently the gate materialis deposited and etched, so that the resulting gate surrounds the fin onthe three exposed sides. The channel region of the device is located inthe fin. To introduce strain into such a strip or fin, researchers haveexplored process-inducement methods similar to those employed insingle-gate MOSFETs, such as, for example, epitaxially raised silicongermanium (SiGe) or silicon carbide (SiC) source and drain regions.There are, however, limitations to such methods, including limitationson how much strain can be introduced into the channel.

SUMMARY

In various embodiments, the invention includes methods and structuresfor introducing strain into FinFET-type devices by exploitingdifferences in natural lattice constants between the channel region inthe fin and a substantially relaxed semiconductor region below thechannel.

Accordingly, an aspect of the invention features a method for forming astructure. The method includes forming a dielectric layer over asemiconductor substrate comprising a first semiconductor material. Atrench is defined in the dielectric layer, extending to a surface of thesubstrate, and having a height h and a width w; the ratio of h to w ispreferably ≧0.5.

A crystalline material is formed in the trench. The crystalline materialincludes (i) a first layer comprising a second semiconductor material,and (ii) a second layer comprising a third semiconductor material. Amajority of dislocation defects in the crystalline material terminatewithin the trench. A portion of the dielectric layer is removed toexpose a side portion of the crystalline material, and a gate is definedover the crystalline material.

One or more of the following features may be included. The second andthird semiconductor materials may have different lattice constants; thedifference between lattice constants may be less than 1%. The secondsemiconductor material may be relaxed and the third semiconductormaterial may be strained. Removing a portion of the dielectric layer mayinclude exposing at least a portion of a sidewall of the second layer.The second semiconductor material may include SiGe, and/or the thirdsemiconductor material may include at least one of Si and Ge.

The second semiconductor material may have a bandgap of at least 0.5 eV.It may include at least one of a III-V compound, such as aluminumantimonide (AlSb), indium aluminum antimonide (InAlSb), galliumantimonide (GaSb), or a II-VI compound, such as cadmium selenide (CdSe),zinc telluride (ZnTe), or cadmium telluride (CdTe).

The third semiconductor material may have a bulk electron mobilitygreater than 2000 cm²/V·s and may include at least one of indiumantimonide (InSb), indium arsenide (InAs), or indium gallium arsenide(InGaAs).

In another aspect, the invention features a method for forming astructure, in which a dielectric layer is formed over a semiconductorsubstrate including a first semiconductor material. A trench is definedin the dielectric layer, extending to a surface of the substrate, andhaving a height h and a width w; the ratio of h to w is preferably ≧0.5.At least one crystalline material layer including a second semiconductormaterial having a lattice mismatch with the first semiconductor materialis formed in the trench, such that a majority of dislocation defects inthe second semiconductor material terminate within the trench. A portionof the dielectric layer is removed to expose a side portion of thecrystalline material, and a gate is defined over the crystallinematerial.

One or more of the following features may be included. The secondsemiconductor material may include at least one of a III-V compound or aII-VI compound. The second semiconductor material may have a bandgap ofat least 0.5 eV and/or have a bulk electron mobility greater than 2000cm²/V·s.

The crystalline material layer(s) may include a first layer comprisingthe second semiconductor material and a second layer comprising a thirdsemiconductor material. The third semiconductor material may include atleast one of a III-V compound or a II-VI compound, and the thirdsemiconductor material may be different from the second semiconductormaterial. The second and third semiconductor materials may havedifferent lattice constants. The second semiconductor material may berelaxed and the third semiconductor material may be strained. The thirdsemiconductor material may include at least one element contained in thesecond semiconductor material. Defining the dielectric layer may includethermal oxidation and/or plasma-enhanced chemical vapor deposition(PECVD).

In yet another aspect, the invention features a method for forming astructure, including removing a portion of a semiconductor substratecomprising a first semiconductor material to define a fin. A dielectriclayer is deposited over the substrate and the fin, and then planarizedsuch that a top surface of the dielectric layer is substantiallyco-planar with a top surface of the fin. A trench is formed by removingat least a portion of the fin, the trench has a height h and a width w,and the ratio of h to w is preferably ≧0.5. A crystalline material isformed in the trench, and includes a second semiconductor materiallattice-mismatched to the first semiconductor material. A majority ofdislocation defects in the crystalline material terminate within thetrench. A portion of the dielectric layer is removed to expose a sideportion of the crystalline material, and a gate is defined over thesecond semiconductor material.

In still another aspect, the invention features a structure having astrained channel and including multiple semiconductor material layers atleast partially disposed in a trench. The structure includes adielectric layer disposed over a semiconductor substrate comprising afirst semiconductor material. A trench is disposed in the dielectriclayer. The trench (i) extends to a surface of the substrate, (ii) isdefined by at least one sidewall having a height h at least equal to apredetermined distance H from the surface of the substrate, and (iii)has a width w. A crystalline material is at least partially disposed inthe trench. The crystalline material includes a first layer comprising asecond semiconductor material and a second layer comprising a thirdsemiconductor material. A side portion of the crystalline materialextends above the dielectric layer, and a gate is disposed over thecrystalline material. The ratio of the height h to the width w is ≧0.5,and a majority of dislocation defects in the crystalline materialterminate at or below the predetermined vertical distance H (with h≧H).The third semiconductor material defines a strained channel.

In another aspect, a semiconductor device includes a transistorstructure formed above a defect trapping region. The semiconductordevice includes a defect-trapping region comprising a trench including adielectric sidewall. The defect-trapping region is proximate a substratehaving a first lattice constant. A semiconductor material, disposedwithin the defect trapping region, has a second lattice constantdifferent from the first lattice constant. A majority of crystallinedefects in the semiconductor material terminate within the defecttrapping region. A fin having a semiconducting sidewall is disposedabove the semiconductor material disposed within the defect trappingregion, and a transistor gate is disposed over the semiconductingsidewall and top surface of the fin.

Still another aspect of the invention features a method for forming astructure. The method includes the steps of forming a dielectric layerover a semiconductor substrate that itself includes a firstsemiconductor material, and defining a trench in the dielectric layerextending to a surface of the substrate. The trench is defined by atleast one sidewall having a height h at least equal to a predetermineddistance H from the surface of the substrate; the trench issubstantially rectangular and has a width w. A crystalline materialincluding a first layer and a second layer is formed in the trench. Thefirst layer includes a second semiconductor material, and the secondlayer includes a third semiconductor material. A portion of thedielectric layer is removed to expose a side portion of the crystallinematerial, and a gate is defined over the crystalline material. The ratioof the height h of the trench to the width w of the trench is ≧0.5,e.g., ≧1, and dislocation defects in the crystalline material terminateat the sidewall of the trench at or below the predetermined distance H(h≧H).

The second and third semiconductor materials may have different latticeconstants. The second semiconductor material may be relaxed and thethird semiconductor material may be strained. Removing a portion of thedielectric layer may include exposing at least a portion of a sidewallof the second layer. The second semiconductor material may compriseSiGe, and the third semiconductor material may include at least one ofSi and Ge.

The second semiconductor material may have a bandgap of at least 0.5 eV.The second semiconductor material may include at least one of a III-Vmaterial and a II-VI material, e.g., at least one of AlSb, InAlSb, GaSb,CdSe, ZnTe, and CdTe.

The third semiconductor material may have a bulk electron mobilitygreater than 2000 cm²/V·s. The third semiconductor material may includeat least one of InSb, InAs, and InGaAs. The difference between a latticeconstant of the second semiconductor material and a lattice constant ofthe third semiconductor material may be less than 1%.

In another aspect, the invention features a method for forming astructure. The method includes removing a portion of a semiconductorsubstrate including a first semiconductor material to define a fin, anddepositing a dielectric layer over the substrate and the fin. Thedielectric layer is planarized such that a top surface of the dielectriclayer is substantially co-planar with a top surface of the fin. A trenchis defined by removing at least a portion of the fin. The trench has atleast one sidewall having a height h at least equal to a predetermineddistance H from the bottom of the trench. The trench is substantiallyrectangular and has a width w. A crystalline material including a secondsemiconductor material is formed in the trench. A portion of thedielectric layer is removed to expose a side portion of the crystallinematerial, and a gate is defined over the second semiconductor material.The ratio of the height h of the trench to the width w of the trench is≧0.5, e.g., ≧1, and dislocation defects in the crystalline materialterminate at the sidewall of the trench at or below the predetermineddistance H, and h≧H.

In another aspect, the invention features a structure including adielectric layer disposed over a semiconductor substrate that itselfincludes a first semiconductor material. A trench is disposed in thedielectric layer and extends to a surface of the substrate. The trenchis defined by at least one sidewall having a height h at least equal toa predetermined distance H from the surface of the substrate. The trenchis substantially rectangular and has a width w. A crystalline materialis at least partially disposed in the trench. The crystalline materialcomprises a first layer including a second semiconductor material and asecond layer including a third semiconductor material. A side portion ofthe second layer extends above the dielectric layer, and a gate isdisposed over the crystalline material. The ratio of the height h of thetrench to the width w of the trench is ≧0.5, e.g., ≧1, dislocationdefects in the crystalline material terminate at the sidewall of thetrench at or below the predetermined distance H, h≧H, and the thirdsemiconductor material defines a strained channel.

The second and third semiconductor materials may have different latticeconstants. The second semiconductor material may be relaxed and thethird semiconductor material may be strained. In some embodiments, thesecond semiconductor material may include SiGe and the thirdsemiconductor material may include at least one of Si and Ge.

In another aspect, the invention features a structure including adielectric layer disposed over a semiconductor substrate that itselfincludes a first semiconductor material. A trench is disposed in thedielectric layer and extends to a surface of the substrate. The trenchis defined by at least one sidewall having a height h at least equal toa predetermined distance H from the surface of the substrate. The trenchis substantially rectangular and has a width w. A crystalline materialis at least partially disposed in the trench. The crystalline materialcomprises a first layer including a second semiconductor layer and asecond layer including a third semiconductor material. A side portion ofthe second layer extends above the dielectric layer, and a gate isdisposed over the crystalline material. The ratio of the height h of thetrench to the width w of the trench is ≧0.5, e.g., ≧1, dislocationdefects in the crystalline material terminate at the sidewall of thetrench at or below the predetermined distance H, h≧H, and the secondsemiconductor material has a bandgap of at least 0.5 eV.

The second semiconductor material may include at least one of a III-Vmaterial and a II-VI material, e.g., at least one of AlSb, InAlSb, GaSb,CdSe, ZnTe, or CdTe. The third semiconductor material may have a bulkelectron mobility greater than 2000 cm²/V·s, and may include, e.g., atleast one of InSb, InAs, and InGaAs. The difference between a latticeconstant of the second semiconductor material and a lattice constant ofthe third semiconductor material may be less than 1%.

BRIEF DESCRIPTION OF FIGURES

In the drawings, like reference characters generally refer to the samefeatures throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIGS. 1-2 are schematic perspective views illustrating a method forformation of a device on a semiconductor substrate in accordance with anaspect of the invention;

FIGS. 3-4 are schematic perspective views illustrating an alternativemethod for formation of a device on a semiconductor substrate inaccordance with other aspects of the invention; and

FIGS. 5-7 are schematic perspective views illustrating anotheralternative method for formation of a device on a semiconductorsubstrate in accordance with another aspect of the invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a substrate 100 includes a first semiconductormaterial S1. The substrate 100 may be, for example, a bulk Si wafer, abulk germanium (Ge) wafer, a semiconductor-on-insulator (SOI) substrate,or a strained semiconductor-on-insulator (SSOI) substrate. The substrate100 may include or consist essentially of the first semiconductormaterial, such as a group IV element, e.g., Ge or Si, a III-V compound,or a II-VI compound. In an embodiment, substrate 100 includes orconsists essentially of (100) Si.

A dielectric layer 110 is formed over the semiconductor substrate 100.The dielectric layer 110 may include or consist essentially of adielectric material, such as silicon nitride (Si₃N₄) or silicon dioxide(SiO₂). The dielectric layer 110 may be formed by any suitabletechnique, e.g., thermal oxidation or PECVD. As discussed below, thedielectric layer may have a thickness t₁ corresponding to a desiredheight h of crystalline material to be deposited in a trench formedthrough the dielectric layer. In some embodiments, the thickness t₁ ofthe dielectric layer 110 may be in the range of, e.g., 25-1000 nm.

A mask (not shown), such as a photoresist mask, is formed over thesubstrate 100 and the dielectric layer 110. The mask is patterned toexpose at least a portion of the dielectric layer 110. The exposedportion of the dielectric layer 110 is removed by, e.g., reactive ionetching (RIE) to define a defect trapping region, e.g., trench 120.Trench 120 extends to a surface of the substrate 100 and may be definedby at least one sidewall 130. The height h of the sidewall 130corresponds to the thickness t₁ of the dielectric layer 110, and may beat least equal to a predetermined vertical distance H from a top surface135 of the substrate, calculated as described below.

It has been observed experimentally that dislocations in a mismatchedcubic semiconductor grown on a Si (100) surface in the near vicinity(e.g., within approximately 500 nm or less) of a vertical dielectricsidewall surface bend toward that surface at approximately 30 degreesthrough 60 degrees. For example, the dislocations may bend toward thatsurface at approximately a 45-degree angle to that surface. Based onthis relationship, the predetermined vertical distance H necessary totrap defects is, typically, approximately equal to a width between ½ wand 2 w, where w is the width of the trench. This range is based on therange of intersection angles of approximately 30 degrees through 60degrees; then, tan(30°)w≦H≦tan(60°)w, which roughly corresponds to ½w≦H≦2w.

The trench may be substantially rectangular in terms of cross-sectionalprofile, a top view, or both, and have a width w that is smaller than alength l of the trench. For example, the width w of the trench may beless than about 500 nm, e.g., about 10-100 nm, and the length l of thetrench may exceed each of w and H. The ratio of the height h of thetrench to the width w of the trench 120 may be ≧0.5, e.g., ≧1.

A crystalline material 140 is formed in the trench 120. The crystallinematerial 140 may include or consist essentially of a group IV element orcompound, a III-V compound, or a II-VI compound. Examples of suitablegroup IV elements or compounds include Ge, Si, SiGe, and SiC. Examplesof suitable 111-V compounds include gallium arsenide (GaAs), galliumnitride (GaN), InAs, InSb, InAlSb, AlSb, indium aluminum arsenide(InAlAs), indium phosphide (InP), and InGaAs. Examples of suitable II-VIcompounds include CdSe, ZnTe, and CdTe.

The crystalline material 140 may be formed by selective epitaxial growthin any suitable epitaxial deposition system, including, but not limitedto, metal-organic chemical vapor deposition (MOCVD),atmospheric-pressure CVD (APCVD), low- (or reduced-) pressure CVD(LPCVD), ultra-high-vacuum CVD (UHCVD), molecular beam epitaxy (MBE), oratomic layer deposition (ALD). In the CVD process, selective epitaxialgrowth typically includes introducing a source gas into the chamber. Thesource gas may include at least one precursor gas and a carrier gas,such as, for example, hydrogen. The reactor chamber may be heated by,for example, RF-heating. The growth temperature in the chamber may rangefrom about 300° C. to about 900° C., depending on the composition of thecrystalline material. The growth system may also utilize low-energyplasma to enhance the layer growth kinetics.

The epitaxial growth system may be a single-wafer or multiple-waferbatch reactor. Suitable CVD systems commonly used for volume epitaxy inmanufacturing applications include, for example, an Aixtron 2600multi-wafer system available from Aixtron, based in Aachen, Germany; anEPI CENTURA single-wafer multi-chamber systems available from AppliedMaterials of Santa Clara, Calif.; or an EPSILON single-wafer epitaxialreactor available from ASM International based in Bilthoven, TheNetherlands.

Dislocation defects 150 in the crystalline material 140 reach andterminate at the sidewalls of the trench 120 in the dielectric material110 at or below the predetermined vertical distance H from the surface135 of the substrate, such that dislocations in the crystalline material140 decrease in density with increasing distance from the bottom portionof the trench 120. Accordingly, the upper portion of the crystallinematerial is substantially exhausted of dislocation defects. Variousdislocation defects such as threading dislocations, stacking faults,twin boundaries, or anti-phase boundaries may thus be substantiallyeliminated from the upper portion of the crystalline material.

A planarization step such as, e.g., CMP may be used to ensure that thetop surface of the crystalline material 140 is substantially co-planarwith the top surface of the dielectric material 110.

Referring to FIG. 2, a portion of the dielectric layer 110 isselectively removed to expose a side portion 200 of the crystallinematerial 140. A height h_(channel) of the exposed side portion 200 maybe equivalent to the thickness of a channel region to be formed.Preferably, the portion of the dielectric layer 110 is removed by anisotropic etch, e.g., a wet etch employing, for example, HF. A dry etchmay also be used, e.g., a plasma etch employing CF₄ and H₂.

The crystalline material 140 may be considered as having two portions: alower portion for trapping dislocation defects and an upper portion fora channel portion of a tri-gate MOSFET incorporating the crystallinematerial 140. The height h of the crystalline material thus has twocomponents: the height h_(trapping) of the lower portion (where defectsare concentrated) and the height h_(channel) of the upper portion (whichis largely free of defects). The height h_(trapping) of the trappingportion may be selected from a range of about ½ w≦h_(trapping)≦2w, toensure effective trapping of dislocation defects. The actual value ofh_(trapping) required may depend upon the type of dislocation defectsencountered, which, in turn, may depend on the materials used, and alsoupon the orientation of the trench sidewalls. In some instances, theheight h_(trapping) need not be greater than that required for effectivedefect trapping, in order to ensure that the dislocation defects aretrapped at a sufficient distance away from the channel portion. In thisway, the deleterious effects of dislocation defects upon deviceperformance are not experienced. For example, h_(trapping) may be, e.g.,10-100 nm greater than required for effective trapping of defects. Forthe channel portion, the height h_(channel) may typically be selectedfrom the range of approximately ½ w≦h_(channel)≦10 w. In some cases, ifh_(channel) is significantly less than ½ w, the device may no longer beconsidered a multi-gate device, i.e., if conduction occurs primarily onthe top surface of the gates disposed over the channel portion. Ifh_(channel) is significantly greater than 10 w, subsequent deviceprocessing may be challenging, for example, because of possiblemechanical instability of the fin during processing, or because ofchallenges in implanting ions in sidewalls of closely packed devicesthat include tall fins.

A gate 210 is defined over the crystalline material 140 by depositionand selective removal of a gate dielectric material 220 and a conductivegate material 230. The gate dielectric material may be, e.g., SiO₂,Si₃N₄, HfO₂, HfSiON, and/or HfSiO. The conductive gate material 230 maybe, for example, polysilicon, amorphous Si, Ge, or SiGe gate material,or a metal or metal alloy. In a tri-gate device, gate 210 has threeportions, one on each exposed side portion of the crystalline material140 and one on a top surface of the crystalline material 140.

A source and a drain may be defined in source and drain regions 240, 250of the crystalline material 140 proximate to the gate 210 and next to achannel 260 disposed under the gate 210. The resulting structure is atri-gate MOSFET 270, employing the benefits of aspect ratio trapping(“ART”). As used herein, “ART” refers generally to the technique(s) ofcausing defects in a material to terminate on a side surface as thematerial grows vertically, e.g., at a dielectric sidewall, with thesidewall being sufficiently high with respect to a width of the growtharea, such that it traps most, if not all, of the defects.

Referring to FIGS. 3 and 4, in some embodiments, the crystallinematerial 140 includes a first layer 300 and a second layer 310. Thefirst layer 300 includes a second semiconductor material S2, and thesecond layer 310 includes a third semiconductor material S3. Each of thesemiconductor material S2, S3 may include or consist essentially of oneor more of a group IV element or compound, a III-V compound, or a II-VIcompound. Examples of suitable group IV elements or compounds includeGe, SiGe, and SiC. Examples of suitable 111-V compounds include GaAs,GaN, InAs, InSb, InAlSb, AlSb, InAlAs, InP, and InGaAs. Examples ofsuitable II-VI compounds include CdSe, ZnTe, and CdTe.

In an embodiment, the second and third semiconductor materials S2, S3may be different materials having different lattice constants. Thelattice mismatch may be selected from a range of, e.g., 0.2-4.0%. If thelattice mismatch is too low, the benefit to performance may not besignificant. If the lattice mismatch is too high, the allowablethickness of the third semiconductor material S3 may be too limited, ifgross relaxation of third semiconductor material S3 is to be avoided.The second semiconductor material S2 may be relaxed and the thirdsemiconductor material S3 may be strained.

In one embodiment, the second semiconductor material S2 includes orconsists essentially of relaxed SiGe, and the third semiconductormaterial S3 includes or consists essentially of Si. This combinationresults in tensile strain in the channel 260. The strain may beprimarily uniaxial in the direction of current flow due to lateralelastic relaxation. This type of strain is particularly beneficial forn-type metal-oxide-semiconductor (NMOS) device performance.

In another embodiment, the second semiconductor material includes orconsists essentially of relaxed SiGe, and the third semiconductormaterial S3 includes or consists essentially of Ge. This combinationresults in compressive strain in the channel 240. The strain may beprimarily uniaxial in the direction of current flow due to lateralelastic relaxation. This type of strain is particularly beneficial forp-type metal-oxide-semiconductor (PMOS) device performance.

The resulting tri-gate MOSFET 270 or FinFET (FIG. 4) includes dielectriclayer 110 disposed over the semiconductor substrate 100 including thefirst semiconductor material S1. Trench 120 is disposed in thedielectric layer 110, extends to the surface 135 of the substrate 100,and is defined by at least one sidewall 130. The sidewall 130 has aheight h at least equal to a predetermined distance H from the surfaceof the substrate. The trench is substantially rectangular and has awidth w. Crystalline material 140 is at least partially disposed in thetrench. The crystalline material comprises a first layer 300 thatincludes a second semiconductor material, and a second layer 310 thatincludes a third semiconductor material. A side portion of the secondlayer extends above the dielectric layer. Gate 210 is disposed over thecrystalline material. A ratio of the height h of the trench to the widthw of the trench is preferably ≧0.5, dislocation defects in thecrystalline material terminate at the sidewall of the trench at or belowthe predetermined distance H, h≧H, and the third semiconductor materialdefines a strained channel 260.

A height h_(s2) of the first layer 300 may be approximately equal toh_(trapping), and a height h_(s3) of the second layer 310 may beapproximately equal to h_(channel), with h_(trapping) and h_(channel)being determined as discussed above with reference to FIG. 2. The heightof the first layer h_(s2) and the height of the second layer h_(o) maybe selected so that the second semiconductor material S2 is relaxed andthe third semiconductor material S3 is strained, or vice versa.

The following relationships may be taken into consideration whenselecting h_(s2) and h_(s3). For compressively strained films,substantial relaxation occurs at a film thickness of approximately 3-4times h_(c), where h_(c) is the critical thickness for the appearance ofmisfit dislocations. For tensilely strained films, substantialrelaxation occurs at a film thickness of about 10h_(c).

Referring still to FIGS. 3 and 4, in another embodiment, the second andthird semiconductor materials S2, S3 may be selected to allow theformation of a bi-layer tri-gate device with high-mobility channelmaterial and suppressed junction leakage. For example, the secondsemiconductor material S2 may have a bandgap of at least 0.5 eV. Thesecond semiconductor material S2 may include or consist essentially of aIII-V material and/or a II-VI material, such as AlSb, InAlSb, GaSb,CdSe, ZnTe, and/or CdTe. The third semiconductor material S3 may have ahigh bulk electron mobility, e.g., greater than 2000 cm²/V·s. Materialsthat are suitable for NMOS and have high mobilities include, e.g., InSb,InAs, and InGaAs.

For some potential III-V channel materials, straining the channel maynot lead to significant performance benefits. In such cases, preferablythe second semiconductor material S2 and the third semiconductormaterial S3 have similar lattice constants. Matching lattice constantshelps avoid significant defect formation at an interface 400 betweenthese two materials, while also allowing a sufficient height h_(s3)(e.g., 20-200 nm) of the second layer 310 to make this layer suitablefor use as a MOS channel. In a preferred embodiment, the differencebetween the lattice constant of the second semiconductor material andthe lattice constant of the third semiconductor material is less than1%.

In forming the tri-gate device 270, a source and a drain are defined insource and drain regions 240, 250, respectively. Preferably, bottomportions of the source and the drain are disposed in secondsemiconductor material S2, such that the relatively high bandgap ofsecond semiconductor material S2 helps limit junction leakage.

Referring to FIGS. 5-7, in some circumstances, from the standpoint ofphotolithography, it may be easier to define a narrow line than a narrowtrench. Accordingly, an alternative process sequence for creating trench120 begins with formation of a mask (not shown) over semiconductorsubstrate 100. A portion of the semiconductor substrate 100 is removedto define a fin 500. Fin 500 may have dimensions substantiallyidentically to the dimensions of trench 120. Dielectric layer 110 isdeposited over the semiconductor substrate 100 and the fin 500. Thedielectric layer 110 is then planarized by, e.g., chemical-mechanicalpolishing (CMP) such that a top surface of the dielectric layer issubstantially co-planar with a top surface 510 of the fin 500.

Trench 120 is defined by removing at least a portion of the fin 500. Thefin may be selectively removed with a wet or dry etch that removessemiconductor material Si with respect to the dielectric layer 110. Forexample, if semiconductor material Si primarily comprises Si, anddielectric layer 110 primarily comprises SiO₂, a dry etch utilizing Cl₂and/or HBr may be used to selectively remove the Si. The etch may be atimed etch, thereby avoiding undercutting a bottom portion of thedielectric layer 110.

As discussed above with respect to FIG. 1, trench 120 is defined by atleast one sidewall having a height h at least equal to a predetermineddistance H from the bottom of the trench. The trench may besubstantially rectangular and have a width w. The ratio of the height hof the trench to the width w of the trench is preferably ≧1, dislocationdefects in the crystalline material terminate at the sidewall of thetrench at or below the predetermined distance H, and h≧H. A crystallinematerial 140 is formed in the trench. The crystalline material includessecond semiconductor material S2.

Referring to FIG. 7, a portion of the dielectric layer 110 is removed toexpose a side portion of the crystalline material. A gate dielectric 220and a gate 210 are formed over the second semiconductor material S2, todefine a FinFET.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

1. A semiconductor structure comprising: a semiconductor fin on asubstrate, the substrate comprising a first crystalline semiconductormaterial, the semiconductor fin comprising a second crystallinesemiconductor material, the second crystalline semiconductor materialbeing lattice-mismatched to the first crystalline semiconductormaterial, the second crystalline semiconductor material adjoining thefirst crystalline semiconductor material at an interface, the interfacehaving a rectangular area, the rectangular area having a width and alength, the length being greater than or equal to the width, thesemiconductor fin comprising a sidewall extending from the interface toa height above the interface, the height being greater than half thewidth, and a majority of dislocation defects in the second crystallinesemiconductor material terminate at or below the height; and a gate onthe sidewall of the semiconductor fin and on a top surface of thesemiconductor fin.
 2. The semiconductor structure of claim 1, whereinthe length is greater than the height.
 3. The semiconductor structure ofclaim 1, wherein the semiconductor fin comprises a third crystallinesemiconductor material over the second crystalline semiconductormaterial.
 4. The semiconductor structure of claim 3, wherein the thirdcrystalline semiconductor material has a different composition from thesecond crystalline semiconductor material.
 5. The semiconductorstructure of claim 3, wherein the third crystalline semiconductormaterial is strained.
 6. The semiconductor structure of claim 1 furthercomprising a first source/drain in the semiconductor fin and a secondsource/drain in the semiconductor fin and on an opposite side of thegate from the first source/drain.
 7. The semiconductor structure ofclaim 1 further comprising a dielectric layer on the substrate, thedielectric having a trench in which the semiconductor fin is disposed.8. The semiconductor structure of claim 7, wherein a dielectric sidewallof the dielectric layer is proximate the sidewall of the semiconductorfin.
 9. The semiconductor structure of claim 7, wherein the dielectriclayer has a dielectric height that extends to a point at or above atrapping height, the trapping height corresponding to a point at orbelow which the majority of dislocation defects in the secondcrystalline semiconductor material terminate.
 10. A method for forming asemiconductor structure, the method comprising: epitaxially growing asemiconductor fin on a semiconductor substrate, the semiconductorsubstrate comprising a first crystalline semiconductor material, thesemiconductor fin being at least partially disposed in a trench in adielectric layer on the semiconductor substrate, the trench having aheight and a width, the height being greater than or equal to half thewidth, the semiconductor fin comprising a second crystallinesemiconductor material lattice-mismatched from the first crystallinesemiconductor material, the second crystalline semiconductor materialadjoining the first crystalline semiconductor material, a majority ofdislocation defects in the second crystalline semiconductor materialterminating within the trench; removing a portion of the dielectriclayer to expose a sidewall of the semiconductor fin; and forming a gateon the sidewall of the semiconductor fin and over a top of thesemiconductor fin.
 11. The method of claim 10, wherein the semiconductorfin comprises a third crystalline semiconductor material over the secondcrystalline semiconductor material, the second crystalline semiconductormaterial and third crystalline semiconductor material having differentlattice constants.
 12. The method of claim 11, wherein the secondcrystalline semiconductor material is relaxed and the third crystallinesemiconductor material is strained.
 13. The method of claim 10, whereinthe removing the portion of the dielectric layer does not remove thedielectric layer below a trapping height, the trapping height being apoint at or below which the majority of dislocation defects in thesecond crystalline semiconductor material terminate.
 14. The method ofclaim 10 further comprising forming a first source/drain in thesemiconductor fin and a second source/drain in the semiconductor finopposite the gate from the first source/drain.
 15. A method for forminga semiconductor structure, the method comprising: forming a substratefin on a semiconductor substrate, the substrate fin being integral tothe semiconductor substrate, the substrate fin having a width and alength at a base, the semiconductor substrate comprising a firstcrystalline semiconductor material; forming a dielectric layer on thesemiconductor substrate; planarizing a top surface of the dielectriclayer and the substrate fin; removing at least a portion of thesubstrate fin to define a trench, the trench having a height, the heightof the trench being equal to or greater than half of the width of thesubstrate fin; forming, in the trench, a second material layercomprising a second crystalline semiconductor material having a latticemismatch with the first crystalline semiconductor material, a majorityof dislocation defects in the second crystalline semiconductor materialterminating within the trench; removing a portion of the dielectriclayer to expose a side portion of the second material layer; anddefining a gate over the second material layer.
 16. The method of claim15, wherein the second material layer comprises a third crystallinesemiconductor material having a lattice mismatch with the secondcrystalline semiconductor material.
 17. The method of claim 16, whereinthe third crystalline semiconductor material is strained.
 18. The methodof claim 15, wherein the removing the portion of the dielectric layerdoes not remove the dielectric layer below a trapping height, thetrapping height being a point at or below which the majority ofdislocation defects in the second crystalline material terminate. 19.The method of claim 15, wherein the length of the substrate fin isgreater than the height of the trench.
 20. The method of claim 15further comprising forming source/drain regions in the second materiallayer.